Over a pot of Earl Grey at a chintzy but comfortable hotel close to Buckingham Palace, Dame Mary Perkins, the founder of Specsavers, recalls a meeting she had with Allan Leighton, the chairman of Royal Mail. "He said, 'You know what you are, mate? Glasses for the masses'," she says, pronouncing it so that it rhymes. "And I said, 'We can't go around saying that', so we say 'It is affordable eyecare for everyone.' But I always laugh." She duly does.

Perkins, 65, is a remarkable success story. She trained as an optician when there were few women in the profession and after graduation took over her father's practice, one room above a bakery in Bristol, where she shared a reception with a doctor across the hall. Along with her husband, Doug, whom she met at university, she built the business into a small chain of 23 shops and sold it to a company for £2m that was eventually swallowed by Dolland & Aitchison.

In 1984, they started again with Specsavers, which has long since left D&A in the shade. It is the largest chain of opticians in Britain, with 39% of the market, expanding into countries including Australia, Sweden, Norway and the Netherlands and generating sales last year of £1.2bn. Gok Wan, the fashion celebrity who endorses the products, apparently calls Perkins his second mum. The Sunday Times estimated that the couple is worth around £500m.

I wonder if she ever has "pinch me" moments. She says not, because she hasn't really changed, and it is easy to believe. She is one of the most unassuming millionaires you are likely to meet. When we discuss whether she would be tempted to float Specsavers, she says no. There is no temptation to take money out of the business?

"Well, if you want the high lifestyle… I mean – I'm not poor now, I was a long time ago," she says. "I have the advantage that if my washing machine breaks I can get a new one, and those are the things that matter to me. My car is 12 years old, but there is only a certain amount of money you can spend." She chuckles again. "I haven't had a holiday…" she is about to say in ages, but then remembers. "I'm going to Mauritius for a week in October." She will be meeting her husband, a policeman's son, halfway. He has been building the business in Australia for the past two years, in what was supposed a six-month stint.

Joint ventures

Perkins's father had worked in a chemist until, due to postwar shortages, the government offered fast-track programmes to train as an optician. He worked on his own, unless his daughter was free to help out in holidays from school or college, and was, she says, a "bit of an oddball". However, without his influence she would never have become an optician. After retiring he moved to Guernsey. When Perkins and her husband sold the first business, and signed a non-compete clause for three years, they followed him there.

The decision to start again was driven partly by deregulation of the professions by the Thatcher government. But this time the couple would do it differently, operating joint ventures with the opticians who would be based in their own shops and own half the business. The optician keeps the profits from the store and Specsavers takes a management fee for marketing, shop fittings, auditing, training, and other support. The idea, she says, was to give the opticians a stake in their business: "People work very much better for themselves." She says they have never had to close a store.

The aim was to change the model of the way opticians work. Previously, she says, "you didn't have big showrooms where you walked in and tried on glasses, and sort of wandered around. You walked in, saw a receptionist, who said take a seat, saw the optician, who tested your eyes and then helped you choose your glasses. It wasn't a very happy experience. So we wanted plenty of choice for people, make it more of a fashion purchase than a grudge, medical purchase. Until then, opticians only saw a few people a day. We wanted a volume business, bring the prices down. I set out to be affordable to everyone."

Glasses at Specsavers start at £25. As the recession approached, the business widened its range at the lower end and increased marketing spending and promotions. The consequence, she says, is that revenues have continued to grow this year and are forecast to hit £1.4bn. But the market is becoming more competitive: Specsavers is now facing the threat of the supermarkets and the recent merger of Boots Opticians and D&A. "We are obviously keeping an eye on them," she says of Boots. "They copy us exactly. It is very disappointing, but is flattering I suppose."

She still does the odd bit of mystery shopping to ensure quality of service. "I just keep a little eye on things. I have different wigs and put glasses on. I go in – not in a tracksuit, exactly, but dressed down with a carrier bag and any old pair of glasses. They have become a bit wise to it over the years. But I have this theory that people over 60 are invisible."

There is still room for expansion in Britain, she says, where Specsavers already has 630 stores. She acknowledges the criticism that all high streets now look the same, but says the world is going that way.

Volume business

Specsavers is based in Guernsey and pays corporation tax there, but profits made in the UK are taxed in the UK. "We have long conversations with tax authorities. They have to come and see that we are actually working in Guernsey and not just a plate on the door. But we have a massive office there, are the biggest employer. I think we worked out last year that we paid several hundred million in tax to the UK Treasury. I think they were a bit surprised about that."

All three of the couple's children have gone into the business. Son John is joint managing director, along with Doug; daughter Cathryn is head of internal audit and the youngest, Juliette, is manager in the Netherlands. "I never dreamt that any of my children would be involved," she says. "It is great that they are, but the business can stand on its own feet and that is how it should be."

She does worry about the grandchildren getting involved. The day we met she had been speaking at a conference on family-owned businesses, and is well aware of the theory that most go wrong in the third generation, often as a result of infighting.

In any case, she has no intention of retiring. "What would you have me do? Sit at home and knit? If you enjoy something, stick with it, and I enjoy working." She has a rigorous fitness regime, though it has left scars, literally – one pale one on her left leg from a knee replacement four years ago and another more visible on the right, from four months ago, which is interrupting her yoga and cycling.

Has she always had to wear glasses? "No, no, just for reading." She pauses, before turning the tables. "How old are you? I am being nosy now." Forty-two. "Oh well – you will be needing reading glasses soon," she says.

"By and large, 99.9% of people after a certain age do need glasses for reading. Yes, one day they'll find a gene that stops us ageing, but they are a little way off yet. They haven't dealt with wrinkles yet, let alone eyes," she adds, with a glint in her own.